In general, molded articles such as plastic, glass and wafer are easily charged by friction etc. because of high electrical resistance, whereby there arise problems that: the charged article causes deterioration in appearance, such as dirt or damage, due to the attraction of dust and the like; an electric shock occurs upon contact of a human body with the charged article; and a defective occurs in the article during manufacturing process.
In order to solve these problems, it is common practice to impart antistatic performance to the molded articles. Some techniques for imparting antistatic performance to the molded articles are known, including (1) the addition of a surfactant to the inside of the molded article; (2) the application of a surfactant or silicon compound to a surface of the molded article; and (3) the modification of a surface of the molded article by a plasma treatment.
In the technique (1), however, there may occur decomposition of the molded article, or separation of a part of the molded article, during molding, and phase separation of the surfactant in the molded article over time. In the technique (2), the antistatic performance can be imparted to the molded article with the application of a small amount of the surfactant but may be easily lost by washing with water or by friction. There is also a need in the technique (2) to apply the surfactant separately after molding the resin product.
It is known that a so-called inner-type antistatic agent added to a plastic resin etc. of the article in the technique (1) has at a surface thereof an active group capable of exerting an antistatic effect and, upon decrease of the active group by friction, dirt or the like, regenerates the active group from the inside to maintain the antistatic effect permanently. As the inner-type antistatic agent can be used by mixing or dispersing into raw material of the plastic resin before polymerization, or into the plastic resin before molding, the use of such an inner-type antistatic agent leads to simplification of manufacturing process. In the case of a non-ionic surfactant, however, it is necessary to use a large amount of the non-ionic surfactant in order to impart antistatic performance. This results in a drawback that the surface properties of the article become deteriorated. In the case of an ionic surfactant, it may be possible to impart antistatic performance even by a relatively small amount of the ionic surfactant. The ionic surfactant does not however get dissolved uniformly and transparently and, even if dissolved temporarily, causes phase separation over time to thereby generate an insoluble matter. Any suitable surfactant has not been found.
By contrast, there is an advantage in the technique (2), in which the surfactant is applied to the surface of the molded article, that the molded article of the plastic resin for use as a substrate does not impair its physical properties and can attain good antistatic performance even by a small amount of the surfactant. The original beautiful appearance of the plastic resin molded article may however become deteriorated in the technique (2). Further, the antistatic effect of the surfactant cannot be regenerated when once lost by friction, washing with water or dissolution etc. It is thus necessary to form a polymer coating or a cured film thereof in order to maintain the antistatic effect of the surfactant over a long time except for the case where it is enough to exert the antistatic effect for a long time during manufacturing.
Although not only antistatic performance but also transparency and evenness are recently often required for various products such as optical discs and videotapes and workpieces in semiconductor manufacturing processes, none of the articles has satisfied these performance characteristics.
As mentioned above, there has been developed no technique for imparting good antistatic performance to the molded article of the plastic resin while maintaining the original properties of the plastic resin.
Herein, antistatic agents for exerting antistatic effects are classified into a type that prevents the conduction of static electricity by the electric conductivity of water adsorbed onto the surface of the article and a type that utilizes the electric conductivity of an ionic structure. As the former type of antistatic agents, nonionic surfactants and amino resins are known. On the other hand, cations such as tertiary ammonium cations and metal cations and anions such as sulfonic acid group and phosphonic acid group are widely known as the latter type of antistatic agents.
Ionic compounds, notably organic salts, are also used as antistatic agents. Examples of organic cations are ammonium cation, sulfonium cation, phosphonium cation and carbo cation, whereas examples of organic anions are conjugate base of carboxylic acid, conjugate base of sulfonic acid, conjugate base of phosphoric acid, conjugate base of sulfoneimide, conjugate base of sulfonyl methane acid, borate anion and aluminate anion. In view of the antistatic performance, there are suitably used ionic compounds each consisting of a metal cation and a carbon-containing organic anion. For instance, a lithium salt of tris(trifluoromethanesulfonyl)methide is reported as an ionic compound in which the anion is an organic anion containing a fluoroalkyl group to show good antistatic performance. (See Patent Document 1.)
A copolymer of an olefinic methide monomer CF2═CFOCF2CF(CF3)OCF2SO2C(Li)(SO2CF3)2 and vinylidene fluoride is disclosed in Patent Document 2. This copolymer is however nearly a perfluoro compound and limited for specific uses because of its solvent solubility, substrate surface adhesion and film forming property.